Crystalline metal-organic frameworks (MOFs) comprise a rapidly growing class of permanently microporous materials.1 They are characterized by low densities, high internal surface areas, and uniformly sized pores and channels. These properties point to a broad range of potential applications, including chemical separations,2 catalysis,3 gas storage and release,4 biological imaging,5 and drug delivery.6 Many of these applications require comparatively large cavities. On the other hand, MOF syntheses typically produce catenated structures, thereby reducing cavity size, increasing density, diminishing vapor-uptake capacity and diminishing gravimetric surface area (See FIG. 1). In most cases non-catenated MOFs are desired, but experimentally catenated structures are often obtained.
Optimal performance in applications depends upon the ability to obtain MOFs having: a) cavities and pores of optimal size, shape, and/or chirality, and b) interior and/or exterior surfaces of suitable chemical composition. Systematic (i.e. predictable) tunability of pore size and, to some extent, surface chemical composition, has indeed been nicely demonstrated for certain families of MOFs.7 For others, however, even minor changes in synthesis conditions or strut composition can lead—seemingly unpredictably—to significant differences in cavity-defining metal-node/organic-strut coordination and/or degree of framework catenation.8 Additionally, certain desirable functional groups may be difficult to incorporate directly into MOFs, either due to thermal instability under materials synthesis conditions9 or because of competitive reaction with intended framework components. Together, these complications can make direct assembly of MOFs that are optimal for specific applications particularly challenging.
An emerging alternative design strategy is to construct robust precursor MOFs and then chemically elaborate their internal and/or external surfaces to impart desired properties. While only a handful of examples has thus far been reported,3f,4e,10 it is clear that the strategy is a powerful one. For example, Wu and co-workers added highly catalytic Ti(IV) sites to the chiral dinapthol-based struts of a pre-formed MOF and subsequently used the MOF to facilitate the enantioselective addition of ZnEt2 to aromatic aldehydes.3f Kaye and Long10e photochemically attached Cr(CO)3 to a benzene dicarboxylate strut in η6 fashion. Wang and Cohen10a were able to modify IRMOF-3 post-synthetically by reacting pendant amines with anhydrides; they subsequently demonstrated that modification could alter the affinities of a simple cubic MOF for various guest molecules.10c Various other efforts have been directed to: a) the introduction of charge-compensating alkali metal cations (potential H2 binding sites11) via strut reduction,4e10g b) surface tailoring of nonporous metallo-salen MOFs via reversible coordination of salen metal sites with chiral ligands and subsequent use of the modified MOFs to accomplish partial separation of the R and S forms of 2-phenylethylalcoho12 and c) “click” based modification13 of alkyne-bearing struts to impart hydrophilicity.10f